The invention concerns an NMR spectrometer comprising a magnet coil system which is disposed in the helium tank of a cryostat, and an NMR probe head which is disposed in a room temperature bore of the cryostat and contains a cooled RF resonator for receiving NMR signals from a sample to be examined, wherein the helium tank and the NMR probe head are cooled by a common, multi-stage, compressor-operated refrigerator.
A device of this type is disclosed in WO 03/023433 A1.
The NMR probe head of an NMR spectrometer is located, together with a measuring device, in the bore of a magnet cryostat. This magnet cryostat contains a superconducting coil which generates the magnetic field required for NMR measurements. The NMR probe head as well as the magnet cryostat must be kept at very low temperatures during operation. The thermal loss caused by thermal conduction and thermal radiation is therefore a problem.
For this reason, a refrigerator is conventionally provided for cooling the NMR probe head. Heat exchangers and a transfer line from the refrigerator to the NMR probe head transfer the cooling power generated by the refrigerator (U.S. Pat. No. 5,889,456). The NMR probe head is supplied with coolant through pumps or compressors via the transfer lines. The cooled components of the probe head are usually at temperatures of 10 to 60 Kelvin. A Gifford-MacMahon cooler (GM) or pulse tube cooler (PT) is e.g. used as refrigerator.
The magnet cryostat of an NMR spectrometer comprises a helium tank which contains the superconducting magnet and liquid helium (4.2 K), one or more radiation shields surrounding the helium tank, an outer vacuum container which is subsequently referred to as the outer shell, and one or more neck tubes which connect the helium tank to the outer shell. The radiation shields may also be containers which are filled with liquid nitrogen (77.3 K) to reduce the heat input into the helium tank. Helium and nitrogen are evaporated by the heat input into the helium tank and on the radiation shield which results from radiation and thermal conduction of the neck tubes and further suspension means. To prevent evaporation of expensive helium and nitrogen, refrigerators (PT or GM coolers) are also used to cool magnet cryostats.
It is thereby also possible to connect the refrigerator to the magnet cryostat via transfer lines. The transfer line between the refrigerator and the cryostat must, however, be highly efficient to transfer the thermal flows with little loss. The structure of this design is less compact and it is therefore not used in modern magnet cryostats.
It has proven to be useful to install the cold finger directly in the magnet cryostat. The cold finger is thereby connected to one or more shields in the cryostat and/or condenses evaporated helium in the helium tank. This method is advantageous in that direct cooling is more efficient than external arrangement of the refrigerator and transport of the coolant via a transfer line. A design of this type is described in U.S. Pat. No. 6,389,821. In this method, more helium is condensed than evaporated. For this reason, part of the cooling power must be compensated for by an electric heating means. In this case, part of the cooling power is also “wasted”, without being used.
WO 03/023433 A1 therefore proposes use of the cold finger of the refrigerator which is installed in the magnet cryostat not only for cooling the cryostat but also for cooling the NMR probe head. The installation of the refrigerator in the magnet cryostat, however, entails considerable disadvantages. The generally magnetic regenerator material is located within the stray field of the NMR magnet coil and therefore generates magnetic disturbances. The vibrations which are caused directly by the refrigerator deteriorate the measuring conditions. Moreover, the limited space within the magnet cryostat prohibits arbitrary positioning of the heat exchangers, which limits the possibility of setting the pre-cooling temperature. For this reason, these devices frequently fail to reach an optimum cooling operation. A considerable part of the input power of the cooler, which is approximately 4 to 8 kW, is still lost in these conventional devices.
It is therefore the underlying purpose of the invention to propose an NMR spectrometer with which the probe head and magnet cryostat are cooled by a common refrigerator, wherein the cooling resources of the refrigerator are optimally utilized while minimizing the disturbances in the working volume caused by the refrigerator.